Porth's Essentials of Pathophysiology, 4e

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Nervous System

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system. Diuretics are used to reduce endolymph fluid volume. A low-sodium diet is recommended in addition to these medications. The steroid hormone prednisone may be used to maintain satisfactory hearing and resolve dizziness. Intratympanic gentamicin therapy has been used for ablation of the vestibular system. 81 This treatment is mainly effective in controlling vertigo and does not alter the underlying pathologic process. Surgical methods include the creation of an endolymphatic shunt in which excess endolymph from the inner ear is diverted into the subarachnoid space or the mastoid (endolymphatic sac surgery), and vestibular nerve section. Advances in vestibular nerve section have facilitated the monitoring of CN VII and CN VIII potentials as a means of preventing hearing damage. Abnormal nystagmus and vertigo can occur as a result of CNS lesions involving the cerebellum and lower brain stem. Central causes of vertigo include brain stem isch- emia, tumors, and the demyelinating effects of multiple sclerosis. When brain stem ischemia is the cause of ver- tigo, it usually is associated with other brain stem signs, such as diplopia, ataxia, dysarthria, or facial weakness. Compression of the vestibular nuclei by cerebellar tumors invading the fourth ventricle results in progressively severe signs and symptoms. In addition to abnormal nystagmus and vertigo, vomiting and a broad-based and dystaxic gait become progressively more evident. In contrast to peripherally generated nystagmus, CNS-derived nystagmus is relatively constant rather than episodic; can occur in any direction rather than being primarily horizontal or rotatory; often changes direction through time; cannot be suppressed by visual fixation; and does result in rapid diminution or “fatigue,” as with peripheral abnormalities. Tests of Vestibular Function Diagnosis of vestibular disorders is based on a descrip- tion of the symptoms, a history of trauma or exposure to agents that are destructive to vestibular structures, and physical examination. Tests of eye movements (i.e., nystagmus) and muscle control of balance and equilib- rium often are used. Electronystagmography (ENG) is a precise and objective diagnostic method of evaluating nystagmic eye movements. Electrodes are placed lateral to the outer canthus of each eye and above and below each eye, and a ground electrode is placed on the forehead. With ENG, the velocity, frequency, and amplitude of spontaneous or induced nystagmus and the changes in these measurements brought by a loss of fixation, with the eyes open or closed, can be quantified. Caloric testing involves elevating the head 30 degrees and irrigating each external auditory canal separately with 30 to 50 mL of ice water. The resulting changes in temperature, which are conducted through the petrous portion of the temporal bone, set up convection currents that mimic Disorders of Central Vestibular Function

the effects of angular acceleration. In an unconscious person with a functional brain stem and intact vestibulo- ocular reflexes, the eyes exhibit a jerk nystagmus lasting 2 to 3 minutes, with the slow component toward the irrigated ear followed by rapid movement away from the ear. With impairment of brain stem function, the response becomes perverted and eventually disappears. An advantage of the caloric stimulation method is the ability to test the vestibular apparatus on one side at a time. The test is never done on a person who does not have an intact eardrum or those who have blood or fluid collected behind the eardrum. Rotational testing involves the use of a motor-driven revolving chair or platform. Unlike caloric testing, rota- tional testing depends only on the inner ear and is unre- lated to conditions of the external ear or temporal bone. Motor-driven chairs or platforms can be precisely con- trolled, and multiple graded stimuli can be delivered in a relatively short period. Testing usually is performed in the dark without visual influence and with selected light stimuli. Eye movements are usually monitored using ENG. A major disadvantage of the method is that both ears are tested simultaneously. The Romberg test is used to demonstrate disorders of static vestibular function. It is done be having a person stand with his or her feet together and arms extended forward so that the degree of sway and arm stability can be observed. The person then is asked to close his or her eyes. Deficiency in vestibular static input is indicated by greatly increased sway and a tendency for the arms to drift toward the side of deficiency. If vestibular input is severely deficient, the subject falls toward the deficient side. Treatment of Vestibular Disorders Depending on the cause, vertigo may be treated pharmaco- logically. There are two types of drugs used in the treatment of vertigo. First are the drugs used to suppress the illusion of motion. These include drugs such as antihistamines (e.g., meclizine, cyclizine, dimenhydrinate, and prometha- zine) and anticholinergic drugs (e.g., scopolamine, atro- pine) that suppress the vestibular system. The second type includes drugs used to relieve the nausea and vomiting that commonly accompany the condition. Antidopaminergic drugs (e.g., phenothiazines) and benzodiazepines com- monly are used for this purpose. Vestibular rehabilitation, a relatively new treatment modality for peripheral vestibular disorders, has met with considerable success. 83 It commonly is done by physical therapists and uses a home exercise program that incorporates habituation exercises, balance retraining exercises, and a general conditioning program. The habituation exercises take advantage of physiologic fatigue of the neurovegetative response to repetitive movement or positional stimulation and are done to decrease motion-provoked vertigo, light- headedness, and unsteadiness. The exercises are selected to provoke the vestibular symptoms. The person moves quickly into the position that causes symptoms, holds the position until the symptoms subside, relaxes, and then repeats the exercise for a prescribed number of times. The exercises usually are repeated twice daily.